This invention relates to novel coating compositions and more specifically to corrosion-resisting coatings which can be applied directly to various substrates particularly metal either as a high or low gloss, self-priming topcoat.
Various surfaces and particularly metal surfaces or substrates require the protection of a coating especially when the surfaces are exposed to a corrosive environment. Metal surfaces of aircraft, for example, are exposed to seawater spray which require protection from corrosion. Specifically, aircraft, e.g., Navy aircraft, are exposed to seawater spray in addition to various acid-forming gases such as sulfur dioxide, carbon dioxide, etc. Moreover, various machinery and equipment in the industrial environments, where fossil fuels are used, need protection against corrosion. In addition, it is important that the coatings be resistant to various chemicals and the weather and also be flexible and have good adhesion characteristics.
Presently, coating systems comprise one or more films, i.e., an undercoat or sealant and a topcoat. Aircraft, for example, traditionally have been coated with high performance protective films generally consisting of an epoxy primer and a polyurethane topcoat. The epoxy primers used on the aircraft are designed to adhere to the metal surface to improve the adhesion of the topcoat and inhibit corrosion. The coating compositions of this invention provides adhesion and corrosion resistance, and also has the required degree of flexibility, the desired optical properties, and a substantial resistance to weather and various chemicals. Moreover, the multi-film coatings generally have a total dry film thickness ranging up to about 0.005 inches, e.g., up to about 10 mils or more which adds considerable weight to the aircraft. Further, it is time consuming to apply multi-coats particularly since there is a drying time between each application. In addition, the removal of a multi-coat system also can be difficult and time consuming and generate high levels of volatile organic (VOC) emissions during the coating operations.
In accordance with this invention, however, the corrosion resistant coating consisting of a single coat comprising an epoxy binder e.g. an epoxy resin such as a bisphenol-A epichlorohydrin condensation product having molecular weights ranging up to about 300,000 or higher in combination with a unique mixture of corrosion-inhibiting metal salts or pigments consisting essentially of an alkaline earth metal or zinc phosphate e.g. zinc-barium phosphate, zinc salts of a benzoic acid or a substituted benzoic acid, and calcium borosilicate. All three of these salts or pigments are essential, in the stated proportions, to provide a high gloss film with the necessary corrosion resistance, and adhesion required of a good coating.
Other pigments and particularly titanium dioxide (TiO.sub.2) including spherical TiO.sub.2 particles e.g. vesiculated beads may be included as a pigment together with the above mentioned pigments. The coating compositions of this invention may be applied, as one coat, directly to various hard surfaces such as metal or organic matrix composites etc. and does not require a topcoat or an undercoat to provide a corrosion-resistant finish with the desired optical properties.